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.\" Copyright (C) 2000 Jason Evans <jasone@FreeBSD.org>.
.\" All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
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.\" $FreeBSD$
.Dd February 15, 2000
.Dt SEM_WAIT 3
.Os
.Sh NAME
.Nm sem_wait ,
.Nm sem_trywait
.Nd decrement (lock) a semaphore
.Sh LIBRARY
.Lb libc_r
.Sh SYNOPSIS
.Fd #include <semaphore.h>
.Ft int
.Fn sem_wait "sem_t *sem"
.Ft int
.Fn sem_trywait "sem_t *sem"
.Sh DESCRIPTION
The
.Fn sem_wait
function decrements (locks) the semaphore pointed to by
.Fa sem ,
but blocks if the value of
.Fa sem
is zero, until the value is non-zero and the value can be decremented.
.Pp
The
.Fn sem_trywait
function decrements (locks) the semaphore pointed to by
.Fa sem
only if the value is non-zero.  Otherwise, the semaphore is not decremented and
an error is returned.
.Sh RETURN VALUES
If successful,
.Fn sem_wait
and
.Fn sem_trywait
return 0.  Otherwise, -1 is returned and
.Va errno
is set to indicate the type of
error.
.Sh ERRORS
.Fn sem_wait
and
.Fn sem_trywait
will fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
.Fa sem
points to an invalid semaphore.
.El
.Pp
Additionally,
.Fn sem_trywait
will fail if:
.Bl -tag -width Er
.It Bq Er EAGAIN
The semaphore value was zero, and thus could not be decremented.
.El
.Sh SEE ALSO
.Xr sem_post 3
.Sh STANDARDS
.Fn sem_wait
and
.Fn sem_trywait
conform to
.St -p1003.1-96 .
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